This invention relates generally to the detection of frequency shift key signals and more specifically to the particular demodulating schemes employed by such receivers. With even greater specificity this invention relates to receiving schemes utilizing coherent mark and space detection.
Frequency shift key signals result when data modulate a carrier by shifting its frequency between two distinct levels known as mark and space tones, or frequencies based upon the status of the data being transmitted. Common methods for demodulating such signals included the use of a discriminator, a single phase locked loop, two narrow band filters followed by amplitude detection, or using two phase locked tone decoders. Both the discriminator method and the single phase locked loop method are quite sensitive to interference as they respond to all frequencies between the mark and space tones plus a range of frequencies that are outside of the mark and space tone range. Techniques utilizing complex narrow band filters become intolerant of overall frequency drift (including doppler shift). In order to reduce the lock up time, when utilizing dual phase locked tone decoders, the phase locked loop bandwidth must be made substantially wider than the width optimum for detection. This wide loop bandwidth renders such dual phase locked tone decoders susceptible to interference.